Direct current (DC) energy sources, such as batteries, are used in many situations where constant alternating current (AC) power is not available. Examples include auxiliary power for vehicles such as semi trucks, automobiles, ambulances, motorcycles, recreational vehicles, boats, and standby generators. Examples further include primary power for forklifts, trolling motors, golf carts, pallet trucks, floor scrubbers, wheel chairs, electric vehicles, and scissor lifts. The primary DC energy source used in such applications is a lead acid battery, typically ranging in nominal voltage from 6 to 48 volts.
In many applications, it is desirable or even critical to monitor the charge level of the energy source so that the user has an indication when the energy source is getting low on voltage or is deviating from a desired voltage range, and needs to be recharged or replaced. The voltage level on the energy source is a measurable indicator of charge of the energy source, and can be monitored with a device commonly known as a "monitor" or a "battery monitor" where the source is a battery. Without a monitor, typically the only inherent indication that a user has that the energy source is in a charged state is whether or not it activates the equipment to which it is attached. Many prior art devices exist which measure the voltage level on a battery or other DC energy source and display the charge level through some means (e.g., bar graph, digit readout, mechanical meter) but these devices generally work only for a particular nominal energy source voltage. That is, a prior art 12 volt DC source monitor will not properly monitor, or even work on, a nominal 24 volt energy source. Further, prior art DC monitors often require supplemental power besides the source being monitored in order to operate. The ability for a prior art monitor to work for a plurality of nominal source voltages generally requires some user interaction, for example, turning a knob to select a voltage, reading a printed conversion table, etc. A normal prior art single-voltage DC energy source monitor is typically constructed of a circuit which essentially is a set of voltage level comparators, with the thresholds for the comparators tied to some reference voltage and the output tied to a display device such as a light emitting diode (LED). This solution is impractical for a multiple-voltage DC monitor because with each type of DC source a complete set of comparators and associated reference thresholds must be included which eventually lead to a large, expensive circuit.
In many applications, it is desirable to monitor the direct current (DC) energy source capacity, aging characteristics, or capability of instantaneous current supply, commonly known as the DC energy sources' "condition" or "state-of-health." The conductance of a battery is a measurable indicator of the "state-of-health", and can be tested and/or monitored with a device commonly known as a "battery monitor."
It is thus desirable that a DC energy source voltage monitor be provided which has the capability to automatically determine nominal voltages of the batteries intended for monitoring by the device, and which is relatively simple to manufacture. In addition, it is desirable to be able to monitor the state of health or battery condition, which can be indicated by conductance of the DC source.